Asynchronous or induction generators have for some time formed the basis for the generation of electricity from wind turbines. They are attractive because of their relative simplicity, inexpensiveness and ruggedness. Various types of induction generators are known but all operate on the same basic principle: a prime mover (such as a turbine blade) delivers mechanical power to rotate a rotor, which rotates in the vicinity of the excited primary windings of a stator. In order to act as a generator, the rotor must be driven above (but only slightly above) its synchronous speed, that is, the rotor must rotate at a frequency which slightly exceeds the frequency of rotation of the magnetic field produced by the stator windings when energised (a so-called “negative slip” condition).
Within the general category of induction generators, different specific constructions and methods of stator winding energisation are possible. One particular construction employs a so-called “squirrel cage” rotor comprising a plurality of (typically aluminium or copper) bars extending in parallel around a longitudinal axis, electrically connected by ringed caps, and defining a cylindrical rotor. The squirrel cage usually has an iron core and rotates within the stator.
The stator windings must be energized for the arrangement to produce electricity. This may be carried out either by excitation from an external source or by self excitation. In a wind turbine (intended for generation of electricity for the grid), the grid itself may supply power to the stator windings, either directly or indirectly. Direct excitation of the windings by the grid has the advantage that electricity is generated at the frequency and voltage imposed by the grid. However it requires also that the turbine rotation speed is tied to the grid frequency so that the generator is more or less inefficient at all but a very narrow range of wind speeds. Mechanical gearboxes have been proposed to alleviate this but these are noisy, complex, expensive and add to the overall mass. Moreover, once the prime mover drives the generator above its generating speed, it ceases generating and ceases resisting the movement of the prime mover; this in turn can result in over-speed and self-destruction. WO-A-94/03970 addresses this problem but requires an expensive, bespoke wound-rotor induction motor rather than the simpler, cheaper squirrel-cage induction motor described above.
Instead of a direct three phase connection from the grid, an electronic system may instead be provided to supply three phase power to the stator windings at a variable voltage and frequency so as to allow the turbine/generator combination to extract power at maximum efficiency over a range of wind speeds: see, for example, “Power electronics for modern wind turbines” by Blaabjerg, published by Morgan and Claypool.
Although such arrangements do provide much more flexibility than the directly grid connected arrangements outlined above, they are however very expensive, complex and difficult to fault find and repair when in situ. Thus the are prohibitive for smaller wind turbines intended for domestic use with an output of, say, 10 kW or so.
As an alternative to external excitation of the stator windings, self excitation by the provision of a local reactive current is also possible. The simplest way to provide such a reactive current is to operate the generator into a passive load having a suitable reactive component, such as a local capacitor bank. The self excitation condition is that the total series impedance of the generator plus its load is zero: that is, both the real and imaginary components of the total impedance are zero. Then, the conductive component of the generator impedance is cancelled by the capacitance of the load impedance and the apparent negative resistance of the generator equals the resistance of the load.
Although self excitation avoids the requirement for a grid connection, on the downside, a capacitatively loaded induction generator that is configured to output a given voltage is capable of generating over only a very narrow band of frequencies above the frequency for which the total impedance of the circuit is zero. Thus, to maintain generation, the frequency of the prime mover must be carefully regulated. One suitable arrangement to address that is shown in GB-A-405,234. Nevertheless, such arrangements suffer from the same disadvantages of the directly grid connected induction generators described above, in that the turbine must rotate at essentially a fixed speed so that either a mechanical gearbox or feathering of the turbine blades must be employed. A further disadvantage with such arrangement is that the output voltage of such a self excited induction generator varies with the torque provided by the prime mover (turbine blade) unless steps are taken to regulate it. US-A-2006/132,103 and U.S. Pat. No. 4,417,194 propose techniques for regulating both voltage and frequency of such a self excited induction generator. However, such arrangements are unsuitable for use with a wind turbine because of the need to control the input torque. Other arrangements of stator windings are contemplated in, for example, U.S. Pat. No. 6,788,031 and US-A-2004/0263110 but again these suffer from cost and complexity of manufacturing.
In principle, the reactive current needed to excite a self excited induction generator can be varied by employing a variable capacitor as the reactive load upon the generator. However, the size of capacitance required for a workable generator is tens or hundreds of microfarads and variable capacitors of this magnitude are impractical. One solution that has been proposed to this practical problem is to use an electronic network that performs active power factor correction (APFC). This relies upon the observation that the reactive current supplied to the generator need not be a perfect sine wave in order for the generator to self excite. Triacs may be used to switch inductive and reactive components in and out of the generator circuit on a timescale comparable with or less than the period of an electrical power cycle. This has the effect of moderating the effective amplitude of the injected reactive current and is thus similar in effect to varying the value of a simple load capacitor. US-A-2006/132103 and U.S. Pat. No. 4,242,628 show examples of this approach. Cost and complexity are once more problems, however.
U.S. Pat. No. 2,758,272 shows a different and more primitive method of implementing power factor correction. Here, the induction generator is operated in parallel with the primary winding of a saturable transformer across the secondary winding of which is connected a fixed reactive load. By varying the DC current through the transformer windings, the saturation of the magnetic core may be controlled for which in turn controls modifies the mutual inductance, and hence in turn the value of the secondary reactance seen reflected at the primary terminals. Although this technique avoids the use of semiconductor power electronics, and thus side steps the choice between cheap but low reliability semiconductors and expensive but more reliable triacs), the expense and complexity of manufacture of a suitably bespoke saturable transformer also means that such an approach is not favoured.
Instead of trying to control multiple variables (torque, reactive current, voltage and/or frequency)—see for example the above referenced US-A-2006/132103—still further prior art arrangements seek to hold constant the ratio of the voltage to the frequency, as described in U.S. Pat. No. 2,922,895. Whilst this may be suitable for the supply of power to induction motors, it is wholly unsuitable for the case of providing electrical power output from a wind turbine.